Screw Compressor

ABSTRACT

A screw compressor includes a compressor body and a shell and tube heat exchanger for cooling of a compressed air discharged from the compressor body. A tube provided in the shell and tube heat exchanger has both ends each having a cylindrical shape, and a center part formed as a multi-lobate tube to have a corrugated shape with crest portions and trough portions alternately arranged in a circumferential direction. The multi-lobate tube provided at the center part of the tube is formed to have a multi-lobate spiral shape through twisting. Size reduction of the shell and tube heat exchanger may make the screw compressor compact as a whole.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. JP 2011-221659, filed on Oct. 6, 2011, the entire contents of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a screw compressor provided with a shell and tube heat exchanger for the purpose of cooling a compressed gas.

BACKGROUND

JP-A No. 2001-153080 discloses the screw compressor provided with the shell and tube heat exchanger as related art. Specifically, the water-cooled two-stage oil-free screw compressor employs the water-cooled shell and tube heat exchangers for the inter-cooler and after-cooler of the compressor. It is configured to cool the compressed air (compressed gas), which has been compressed and brought into high temperature by the one-stage compressor body and the two-stage compressor body using cooling water.

SUMMARY

JP-A No. 2001-153080 does not sufficiently consider with respect to compact structure of the inter-cooler or the after-cooler each formed as the shell and tube heat exchanger. The size of the screw compressor as a whole is increased by employing the large shell and tube heat exchanger. Especially, the screw compressor which includes the inter-cooler and the after-cooler as the shell and tube heat exchangers together with the compressor body housed in a single package is enlarged, resulting in increased volume occupation rate.

It is an object of the present invention to provide a screw compressor that is compact as a whole by reducing the size of the shell and tube heat exchanger.

The present invention provides a screw compressor that includes a compressor body, and a shell and tube heat exchanger for cooling of a compressed air discharged from the compressor body. A tube provided in the shell and tube heat exchanger has both ends each having a cylindrical shape, and a center part formed as a multi-lobate tube to have a corrugated shape with crest portions and trough portions alternately arranged in a circumferential direction.

Preferably, the shell and tube heat exchanger includes a cylindrical shell, headers provided at both ends of the shell, a flange attached to one end of the shell to separate an inside of the shell from one of the headers fluid-tightly, a tube plate attached to the other end of the shell to separate the inside of the shell from the other of the headers fluid-tightly, a plurality of the tubes attached to the flange and the tube plate so as to communicate with the headers provided at the both ends of the shell, and a plurality of baffle plates provided at intervals in the shell in a longitudinal direction, which have through holes through which the tubes penetrate, and guide a fluid introduced into the shell from the one side to the other side of the headers while forming a meander flow of the fluid.

Preferably, an inner diameter of the through hole formed in the baffle plate is larger than the outer diameter of the part of the multi-lobate tube in a state where an internal pressure acts on an inside of the tube by operating the compressor.

Preferably, a part of the multi-lobate tube provided at the center part of the tube is formed to have a multi-lobate spiral shape through twisting.

An outer diameter of the part of the multi-lobate tube of the tube may be smaller than each outer diameter of cylindrical parts at both ends of the tube.

The present invention ensures improved heat exchange efficiency of the shell and tube heat exchanger, and accordingly, reduces its size. This may provide the effect of realizing the screw compressor which can be made compact.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The drawing figures depict one or more implementations in accord with the present teachings, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements.

FIG. 1 is a view representing a general structure of a screw compressor according to a first example of the present invention;

FIG. 2 is a longitudinal sectional view showing a structure of a shell and tube heat exchanger as shown in FIG. 1;

FIG. 3 is a sectional view taken along line A-A of FIG. 2;

FIG. 4 is a sectional view taken along line B-B of FIG. 2;

FIG. 5 is a front view showing a tube employed in the shell and tube heat exchanger;

FIG. 6 is a sectional view taken along line C-C of FIG. 5;

FIG. 7 is a sectional view taken along line D-D of FIG. 5;

FIG. 8 is a perspective view showing an enlarged part of a multi-lobate tube shown in FIG. 5;

FIG. 9 is a perspective view showing an enlarged structure of one end of the tube shown in FIG. 5; and

FIG. 10 is an explanatory view of the flow of the fluid in the shell and tube heat exchanger shown in FIG. 2.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings.

Examples of the screw compressor according to the present invention will be described referring to the drawings. The same parts will be designated with the same codes in the respective drawings.

FIG. 1 is a view showing a general structure of the screw compressor as a first example of the present invention. The screw compressor according to the example is formed as the water-cooled two-stage oil-free screw compressor which is provided with a low stage (first stage) compressor body 2 and a high stage (second stage) compressor body 3 driven and rotated by a main motor 1. The compressed air that has been compressed and brought into high temperature by the low stage compressor body 2 is cooled by an inter-cooler (low pressure stage heat exchanger) 4. The cooled compressed air is fed to the high stage compressor body 3 where it is further compressed. The compressed air having the temperature brought to be high again through compression in the high stage compressor body 3 is cooled by an after-cooler (high pressure stage heat exchanger) 5. It is then discharged outside a package 16 that houses the aforementioned devices.

The compressor bodies 2 and 3 are connected to the main motor 1 via a gear box 20. The gear box 20 houses a bull gear 21 mounted to a leading end of a rotary drive shaft of the main motor 1 and pinion gears 22 and 23 which are mounted to the respective leading ends of rotary driven shafts of the compressor bodies 2 and 3, and in mesh with the bull gear 21. The rotating force generated by rotation of the main motor 1 is transmitted to the compressor bodies 2 and 3 via the bull gear 21 and the pinion gears 22 and 23. The compressor bodies 2 and 3 are then rotated to compress the compression air.

The inter-cooler 4 is connected to a discharge side of the low stage compressor body 2 via an air passage 24, and is further connected to an intake side of the high stage compressor body 3 via an air passage 25. The after-cooler 5 is connected to a discharge side of the high stage compressor body 3 via an air passage 26 and a check valve 27. Cooling water (refrigerant) is supplied to the inter-cooler 4 and the after-cooler 5 so as to cool the compressed air that has been compressed and brought into high temperature by the low stage compressor body 2 and the high stage compressor body 3.

An air discharge piping 29 is diverged from the air passage 26 before the check valve 27. A blower cooler 30 is formed of a copper U-tube and connected to the air discharge piping 29. An air discharge piping 31 is provided downstream of the blower cooler 30. The downstream side of the air discharge piping 31 is connected to an air discharge valve 33 that is operated in association with a suction throttle valve 32. An unloader 34 is formed of the suction throttle valve 32 and the air discharge valve 33. A blower silencer 35, a suction filter 36, and a cooling fan 37 are also provided.

An oil pump 38 serves to circulate the oil within the gearbox 20. An oil cooler 39 is provided for cooling the oil.

In the above-described water-cooled two-stage oil-free screw compressor, when the low stage compressor body 2 and the high stage compressor body 3 are driven to rotate by the motor 1, the compression air is sucked from outside into the low stage compressor body 2 via the suction throttle valve 32. The compressed air that has been compressed and brought into high temperature is discharged from the low stage compressor body 2, and flows into the inter-cooler 4 through the air passage 24 so as to be cooled. The cooled compressed air is sucked into the high stage compressor body 3 through the air passage 25 so as to be further compressed therein. The compressed air discharged from the high stage compressor body 3 flows into the after-cooler 5 via the air passage 26 and the check valve 27. As the compressed air compressed by the high stage compressor body 3 has been brought into high temperature again, it is cooled by the after-cooler 5 to an actual service temperature (for example, the temperature close to that of the sucked atmosphere) so as to be discharged outside the package 16.

The two-stage oil-free screw compressor generally employs the inter-cooler 4 that decreases the temperature of air flowing into the high stage compressor in order to prevent excessive temperature rise in the compressed air discharged from the high stage compressor body 3, and to improve the compression efficiency of the high stage compressor body 3.

Each of the inter-cooler 4 and the after-cooler 5 is formed of the shell and tube heat exchanger for cooling the discharged compressed air to the actual service temperature. Those heat exchangers are housed in the single housing together with the compressor body as a package. The volume occupation rate of the heat exchanger in the resultant package 16 becomes considerably high. The structure of the shell and tube heat exchanger will be described referring to FIG. 2.

The shell and tube heat exchanger includes a cylindrical shell 6, headers 7 and 8 that are attached to the respective ends of the shell 6, a flange 9 attached to one end of the shell 6, which fluid-tightly separates an inside of the shell 6 from one of the headers 7 and 8, a tube plate 10 attached to the other end of the shell 6, which fluid-tightly separates the inside of the shell 6 from the other of the headers 7 and 8, a plurality of tubes 11 provided for the flange 9 and the tube plate 10 so as to communicate the headers 7 and 8 attached to the respective ends of the shell 6, a plurality of baffle plates 14 which are longitudinally arranged at intervals within the shell 6, and have through holes (not shown in the drawing) through which the tubes 11 penetrate, an inlet 12 provided at the other end of the shell 6 for introduction of the fluid thereinto, and an outlet 13 provided on the shell 6 at the side opposite the inlet 12 for discharge of the fluid introduced into the shell 6. The baffle plate 14 serves to guide the fluid introduced through the inlet 12 into the shell 6 from one side (in this example, the tube plate 10 side) to the other side (in this example, the flange 9 side) while meandering.

Each flow of the compressed air and the cooling water resulting from the use of the shell and tube heat exchanger as the after-cooler 5 shown in FIG. 1 will be described referring to FIG. 10.

The compressed air discharged from the high stage compressor body 3 and introduced through an inlet portion 7 a into the header 7 at one end of the shell and tube heat exchanger via the air passage 26 and the check valve 27 flows into each of a plurality of tubes 11 from a part of the flange 9, and further flows to the header 8 at the other side so as to be converged, and then discharged from an outlet portion 8 a. Meanwhile, the cooling water (fluid as the refrigerant) is introduced into the shell 6 from the inlet 12 thereon, and flows toward the flange 9 while meandering through the baffle plates 14 from the tube plate 10 in the shell 6 so as to be discharged from the outlet 13. The cooling water flows while forming a flow channel defined by the baffle plates 14, which is perpendicular to the horizontally arranged tubes 11.

The compressed air and the cooling water become countercurrents with each other so as to exchange heat between the high temperature compressed air and the low temperature cooling water via each wall of the tubes 11. The compressed air is cooled to the actual service temperature at which no problem occurs when it is discharged outside. It is then discharged from the outlet portion 8 a of the header 8, and supplied outside the package.

This applies to the use of the shell and tube heat exchanger as the inter-cooler 4.

Referring back to FIG. 2, a simply cylindrical-shaped single tube (bare tube) has been generally used as the tube 11 employed in the shell and tube heat exchanger. Use of the simply cylindrical-shaped single tube with small heat transfer area may enlarge the shell and tube heat exchanger. Housing of the inter-cooler 4 and the after-cooler 5 each formed as the shell and tube heat exchanger together with the compressor bodies 2 and 3 in the single housing into a package may considerably increase size of the screw compressor as a whole.

For the purpose of improving heat exchange performance of the tube 11, employment of the duplex tube as disclosed in JP-A No. 2008-232449 has been evaluated. The disclosed heat exchanger of duplex tube type is formed of an outer pipe (outer tube) and an inner pipe (inner tube). The outer pipe is a bare tube (cylindrical tube), and the inner tube is formed of a multi-lobate tube with a multi-lobate cross section. A first fluid is allowed to flow through a gap between the inner pipe and the outer pipe, and a second fluid is allowed to flow through the inner pipe. The use of the multi-lobate tube as the inner pipe aims at improvement of the heat exchange efficiency.

The disclosed duplex tube (heat exchanger) as the above-described related art is applied to the tube in the shell and tube heat exchanger so that the compressed air is allowed to flow not only through the passage inside the inner pipe but also the passage between the inner pipe and the outer pipe from the compressor body 2 or 3, and that the cooling water is allowed to flow outside the outer pipe. It is thought that the aforementioned structure using the duplex tube provides the larger heat transfer area and improves the heat exchange efficiency compared to the generally employed structure that employs the single tubes each with the same diameter as the tubes 11 in the shell and tube heat exchanger.

Even if the tube 11 formed as the single tube in the shell and tube heat exchanger is replaced with the duplex tube, the contact area between the tube 11 and the fluid outside the tube (cooling water) differs little from that of the single tube with the same diameter. Sufficient heat exchange efficiency of the shell and tube heat exchanger cannot be highly expected in spite of employment of the duplex tube with complicated structure.

Referring to FIGS. 3 to 9, the present example employs the multi-lobate tube having both ends cylindrically shaped, and the corrugated center part with crest portions and trough portions alternately arranged in the circumferential direction as the tube 11. In this example, the single tube is employed for forming the multi-lobate tube as the tube 11 rather than the duplex tube.

FIG. 3 is a sectional view taken along line A-A of FIG. 2. Referring to the drawing, the tube 11 is formed of the multi-lobate tube. As FIG. 3 shows, the tube 11 is provided to penetrate through a through hole 14 a of the baffle plate 14. The inner diameter of the through hole 14 a is made larger than the outer diameter of the penetrating tube 11 with the multi-lobate shape. Especially when the compressed air flows through the tube 11 upon operation of the compressor, the internal pressure acts on the multi-lobate portion (for example, the internal pressure of 0.7 MPa) to increase the outer diameter. This example is configured so that the inner diameter of the through hole 14 a is made larger than the outer diameter of the multi-lobate portion in the state where the internal pressure acts on the inside of the tube upon operation of the compressor.

The aforementioned configuration allows easy insertion of the tube 11 into the through hole 14 a of the baffle plate 14. Even if the internal pressure acts on the tube 11 upon operation of the compressor to increase the outer diameter of the tube, the configuration prevents generation of stress to the baffle plate 14. Damage to the baffle plate 14 may be prevented, thus allowing the use of the thinner baffle plate.

The baffle plate 14 is used to circulate the cooling water in a meandering manner, and accordingly, is configured to have a shape with the notch at the lower portion or the upper portion rather than the circular shape. The baffle plate 14 is positioned and fixed by a tie rod (not shown in the drawing) fixed to the flange 9 and the tube plate 10.

FIG. 4 is a sectional view of a part of the flange 9, taken along line B-B of FIG. 2. The flange 9 is interposed between the member that constitutes the shell 6 and the member that constitutes the header 7, and is tightened with bolts 15 and the like. The flange 9 has a plurality of through holes 9 a through which the tubes 11 are inserted and fixed. Each one end of the tubes 11 is inserted through the through hole, and expanded using a tube expander so that the end portion of the tube 11 is crimped and fixed in the through hole 9 a of the flange 9. Alternatively, the tube may be fixed through brazing.

The configuration of the tube 11 will be described in detail referring to FIGS. 5 to 9.

FIG. 5 is a front view showing a general structure of the tube 11 used in the shell and tube heat exchanger shown in FIG. 2. Referring to the drawing, the tube 11 has both ends formed as cylindrical parts 11 a and 11 b, and the center part formed as a multi-lobate tube 11 c. That is, as FIG. 6 that is the sectional view taken along line C-C of FIG. 5 illustrates, the center part of the tube 11 is formed of the multi-lobate tube 11 c configured to form a corrugated shape with crest and through portions alternately arranged in the circumferential direction. In this example, the multi-lobate tube 11 c is formed to have five lobe portions. As FIG. 7 that is the sectional view taken along line D-D of FIG. 5 illustrates, the both ends of the tube 11 are formed of simply structured cylindrical parts 11 a and 11 b.

The cylindrical part 11 a of the tube 11 at the right end side is inserted into the through hole 9 a of the flange 9 and fixed. The cylindrical part 11 b of the tube 11 at the left end side is inserted into a through hole 10 a of the tube plate 10 and fixed through tube expansion or brazing likewise the side of the flange 9. The outer diameter of a part of the multi-lobate tube 11 c of the tube 11 is made smaller than each outer diameter of the cylindrical parts 11 a and 11 b so as to allow easy assembly work of inserting the tubes 11 into the through holes 9 a and 10 a of the flange 9 and the tube plate 10, respectively.

The tube plate 10 is configured to axially slide in the shell 6 or the member that constitutes the header 8 so as to absorb heat expansion of the tube in the axial direction. A seal member 17 is provided between the outer peripheral surface of the tube plate 10 and the inner surface of the shell 6 or the inner surface of the member that constitutes the header 8 so as to seal the space therebetween fluid-tightly.

FIG. 8 is an enlarged perspective view of a part of the multi-lobate tube 11 c of the tube 11. Referring to FIGS. 8 and 5, in this example, the part of the multi-lobate tube 11 c has a spiral shape formed through twisting at a constant angle circumferentially along the axial direction as R in the drawing shows. The part of the multi-lobate tube 11 c is configured to have the spiral shape by setting a die for formation of the multi-lobate tube 11 c in the cylindrical single tube (bare tube), and fixing the die in the axial direction while allowing free rotation in the circumferential direction. The multi-lobate shaped part is formed in the single tube by pulling out the cylindrical single tube from the aforementioned state. The die is positioned only in the axial direction so as to be freely rotatable in the circumferential direction. The die is further configured to rotate in the circumferential direction in association with the operation for pulling out the single tube, thus providing the spirally shaped multi-lobate tube. The degree (twisting angle R) of the twisting conducted on the spiral multi-lobate tube is adjustable by changing the speed at which the single tube is pulled out.

FIG. 9 is an enlarged perspective view of the end portion of the tube 11. Although FIG. 9 only shows the left end of the tube 11, the right end of the tube is similarly structured. Referring to the drawing, the end portion of the tube 11 is kept as the cylindrical part lib (11 a), that is, as the bare tube. The region from the point at which the die is set to the point at which the pull-out operation is finished is formed as the multi-lobate tube 11 c.

The tube 11 has both ends formed as the cylindrical parts 11 a and 11 b, which are inserted into the through holes 9 a and 10 a of the flange 9 and the tube plate 10, respectively, and allowed to be fixed thereto through tube expansion or brazing. As the center part of the tube 11 is formed as the multi-lobate tube 11 c, the heat transfer area of the tube 11 may be increased, thus improving the heat transfer efficiency. This makes it possible to efficiently cool the high temperature compressed air that flows inside the tube 11 using the cooling water (refrigerant) that flows outside the tube 11. As a result, the shell and tube heat exchanger may be made compact.

Each groove part (trough portion shown in FIG. 6) of the multi-lobate tube 11 c has both end portions kept cylindrical so as to serve as dams. If the compressor is stopped in the state where water is accumulated in the grooves of the multi-lobate tube 11 c, and supply of the cooling water is stopped, water is kept accumulated in the grooves, which may cause a risk of corrosion. In this example, the part of the multi-lobate tube 11 c is twisted into the spiral shape. The accumulated water in the grooves of the multi-lobate tube will flow downward and drop under self weight along the spiral grooves. This may prevent accumulation of water in the groove, thus preventing corrosion of the part of the multi-lobate tube.

During operation of the compressor, the cooling water flows outside the multi-lobate tube 11 c. Because of the spiral shape of the multi-lobate tube 11 c, the flow of the cooling water outside the tube 11 and the flow of the compressed air passing inside the tube 11 are brought into the turbulent state. This may ensure further improvement in the heat transfer efficiency.

In this example, the part of the multi-lobate tube 11 c is configured to have the spiral shape. As the twisting angle R is increased, the heat transfer area may be enlarged. The part of the multi-lobate tube 11 c does not have to be spirally shaped. It may have the straight shape at the twisting angle R set to zero. The straight shaped multi-lobate tube may be obtained by conducting the pull-out process of the single tube while inhibiting the circumferential rotation of the die. The straight shape of the multi-lobate tube may reduce the heat transfer area. However, it ensures reduction of the pressure loss of the cooling water that flows outside the multi-lobate tube 11 c.

Table 1 represents experimental results of heat exchange performance of the tube 11 in the shell and tube heat exchanger by making a comparison between the use of the multi-lobate duplex tube as disclosed in JP-A 2008-232449 and the use of the tube in the shell and tube heat exchange according to the example.

As the present example employs the single tube for the tube 11, the heat transfer area becomes smaller than that of the duplex tube. However, the use of the multi-lobate tube makes it possible to significantly increase the contact area with the cooling water. Accordingly, the heat exchange amount per unit heat transfer area may be increased.

According to the example, the shell and tube heat exchanger may be made compact on equal terms with the use of the multi-lobate duplex tube. As the example employs the single tube, the configuration is simplified, which makes manufacturing easier, and considerably reduces the material costs for manufacturing of the tubes 11.

TABLE 1 MULTI-LOBATE PRESENT ITEM UNIT DUPLEX TUBE EXAMPLE WHOLE LENGH mm 1000 1000 OUTER mm Ø19 Ø19 DIAMETER LENGTH OF mm 900 900 MULTI-LOBATE PORTION DIMENSION OF mm Ø31.7 Ø31.7 MATERIAL FOR FORMING MULTI-LOBATE PORTION CROSS-SECTION mm² 170.9 121.9 AREA HEAT TRANSFER mm² 135254 90112 AREA HEAT TRANSFER — 100%  67% AREA RATIO CONTACT AREA mm² 17100 28530 WITH COOLING WATER NUMBER OF TUBES 31 31 COOLER TUBES COMPRESSED MPa 0.21 0.21 AIR PRESSURE COMPRESSED m³/min 18 18 AIR FLOW RATE COOLING WATER L/min 50 50 FLOW RATE COMPRESSED ° C. 141 141 AIR TEMPERATURE AT COOLER INLET COMPRESSED ° C. 30.3 32.5 AIR TEMPERATURE AT COOLER OUTLET COOLING WATER ° C. 19.1 19.1 TEMPERATURE AT INLET HEAT EXCHANGE kcal/h 31563 30936 AMOUNT HEAT EXCHANGE kcal/h 1018 998 AMOUNT PER TUBE HEAT EXCHANGE kcal/h/mm² 0.0075 0.0111 AMOUNT PER UNIT HEAT TRANSFER AREA COMPARISON — 100% 147% WITH RESPECT TO HEAT EXCHANGE EFFICIENCY

In the example, the case with respect to the water-cooled two-stage oil-free screw compressor has been described. However, the example may be similarly implemented by using any type of the screw compressor, for example, the single-stage oil-free screw compressor and the oil-cooled screw compressor so long as the shell and tube heat exchanger is used for cooling of the compressed air.

As described above, in a screw compressor that includes a compressor body and a shell and tube heat exchanger for cooling of a compressed air discharged from the compressor body, a tube provided in the shell and tube heat exchanger has both ends each having a cylindrical shape, and a center part formed as a multi-lobate tube to have a corrugated shape with crest portions and trough portions alternately arranged in a circumferential direction. This makes it possible to improve the heat exchange efficiency of the shell and tube heat exchanger, resulting in the compact structure. Accordingly, the screw compressor may be compact as a whole.

The tube provided in the shell and tube heat exchanger is formed of the single tube with the multi-lobate tube part. Compared to the use of the duplex tube, the screw compressor that employs the compact shell and tube heat exchanger with a significantly simple configuration, and improved cooling performance may be realized.

The center part of the tube has the multi-lobate spiral shape through twisting so as to further increase the heat transfer area inside/outside the tube, and to bring both fluids flowing inside/outside the tube into the turbulent state, thus further improving the cooling capability by improving the heat exchange performance. If the fluid flowing outside the tube is liquid such as water, there may be the risk of accumulation of the liquid in the lobe-like groove of the multi-lobate tube when the compressor is stopped or the compressor is not used. In this example, the multi-lobate tube part has the spiral shape, thus preventing accumulation of the liquid as the fluid in the lobe grooves. This makes it possible to prevent generation of rust and corrosion in the tube.

The present example provides the screw compressor provided with the shell and tube heat exchanger that is compact, highly reliable, simply configured, and allows manufacturing at lower costs.

While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings. 

What is claimed is:
 1. A screw compressor comprising: a compressor body; and a shell and tube heat exchanger for cooling of a compressed air discharged from the compressor body, wherein a tube provided in the shell and tube heat exchanger has both ends each having a cylindrical shape, and a center part formed as a multi-lobate tube to have a corrugated shape with crest portions and trough portions alternately arranged in a circumferential direction.
 2. The screw compressor according to claim 1, wherein: the shell and tube heat exchanger includes a cylindrical shell, headers provided at both ends of the shell, a flange attached to one end of the shell to separate an inside of the shell from one of the headers fluid-tightly, a tube plate attached to the other end of the shell to separate the inside of the shell from the other of the headers fluid-tightly, a plurality of the tubes attached to the flange and the tube plate so as to communicate with the headers provided at the both ends of the shell, and a plurality of baffle plates provided at intervals in the shell in a longitudinal direction, which have through holes through which the tubes penetrate, and guide a fluid introduced into the shell from the one side to the other side of the headers while forming a meander flow of the fluid.
 3. The screw compressor according to claim 1, wherein a part of the multi-lobate tube provided at the center part of the tube is formed to have a multi-lobate spiral shape through twisting.
 4. The screw compressor according to claim 1, wherein an outer diameter of the part of the multi-lobate tube of the tube is smaller than each outer diameter of cylindrical parts at both ends of the tube.
 5. The screw compressor according to claim 2, wherein an inner diameter of the through hole formed in the baffle plate is larger than the outer diameter of the part of the multi-lobate tube in a state where an internal pressure acts on an inside of the tube by operating the compressor. 